1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system which controls the air-fuel ratio of an air-fuel mixture supplied to the engine, based on outputs from upstream air-fuel ratio-detecting means and downstream air-fuel ratio-detecting means arranged in the exhaust system at respective locations upstream and downstream of a catalytic converter in the exhaust system of the invention.
2. Prior Art
There has been conventionally known an air-fuel ratio control system for internal combustion engines, for example, from Japanese Provisional Patent Publication (Kokai) No. 63-195351, in which a so-called double O2 sensor system is employed. According to the proposed air-fuel ratio control system, in controlling the air-fuel ratio of a mixture supplied to the engine to a desired value in a feedback manner responsive to an output from an upstream O2 sensor as air-fuel ratio-detecting means arranged in the exhaust system at a location upstream of a catalyst in the exhaust system, when the output from the upstream O2 sensor is inverted with respect to a predetermined value, a skip amount (proportional term) is added to or subtracted from an air-fuel ratio correction coefficient. The skip amount to be added or subtracted is changed based on an output from a downstream O2 sensor as air-fuel ratio-detecting means arranged downstream of the catalyst. Further, a calculation is made of the difference between the output from the downstream O2 sensor and a predetermined reference value corresponding to a stoichiometric air-fuel ratio, and an amount of change per unit time for updating the skip amount is increased as the calculated difference is larger.
However, the above proposed air-fuel ratio control system only executes integral control by progressively decreasing or increasing the skip amount after the output VO2R from the downstream O2 sensor has crossed the predetermined reference value. As a result, a responsive lag occurs in the air-fuel ratio feedback control based on the output VO2R from the downstream O2 sensor. FIG. 1 shows the relationship timing between the air-fuel ratio A/F of a mixture supplied to the engine, which is calculated by the conventional feedback control system, and the output VO2R from the downstream O2 sensor. As shown in FIG. 1, although an average value of the air-fuel ratio A/F of the mixture, i.e. the air-fuel ratio downstream of the catalyst sensed by the downstream O2 sensor should show a value in the vicinity of the stoichiometric value at a time point immediately before an inversion of the output VO2R from the downstream O2 sensor (regions i and j), there unfavorably occurs an over-lean state (region i) or an over-rich state (region j) of the mixture supplied to the engine due to the response lag of the feedback control, since the skip amount (proportional term) to be added to or subtracted from the air-fuel ratio correction coefficient KO2 is only integral-controlled, which results in unfavorably degraded exhaust emission characteristics of the engine.